The glacier used to stretch six square miles (15 square kilometers), Sigurdsson said. Residents reminisced about drinking pure water thousands of years old from Ok.

"The symbolic death of a glacier is a warning to us, and we need action," former Irish president Mary Robinson said.

This was Iceland's first glacier to disappear. But Sigurdsson said all of the nation's ice masses will be gone in 200 years.

"We see the consequences of the climate crisis," Icelandic Prime Minister Katrín Jakobsdóttir said. "We have no time to lose."

Jakobsdottir said she will make climate change a priority when Nordic leaders and German Chancellor Angela Merkel meet in Reykjavik on Tuesday.

"I know my grandchildren will ask me how this day was and why I didn't do enough," said Gunnhildur Hallgrimsdottir, 17.

The plaque, which notes the level of heat-trapping carbon dioxide, also bears a message to the future: "This monument is to acknowledge that we know what is happening and what needs to be done. Only you know if we did it."

After about 100 people made a two-hour hike up a volcano, children installed a memorial plaque to the glacier, nicknamed "OK."

This was Iceland's first glacier to disappear. But Sigurdsson said all of the nation's ice masses will be gone in 200 years.

Icelandic Prime Minister Katrín Jakobsdóttir called the glacial loss a consequence of the climate crisis.

This combination of Sept. 14, 1986, left, and Aug. 1, 2019 photos provided by NASA shows the shrinking of the Okjokull glacier on the Ok volcano in west-central Iceland. A geological map from 1901 estimated Okjökull spanned an area of about 38 square kilometers (15 square miles). In 1978, aerial photography showed the glacier was 3 square kilometers. in 2019, less than 1 square kilometer remains. (NASA via AP)